Many hydrocarbon wells are unable to produce at commercially viable levels without assistance in lifting the formation fluids to the earth's surface. In some instances, high fluid viscosity inhibits fluid flow to the surface. More commonly, formation pressure is inadequate to drive fluids upward in the wellbore. In the case of deeper wells, extraordinary hydrostatic head acts downwardly against the formation and inhibits the unassisted flow of production fluid to the surface.
A common approach for urging production fluids to the surface uses a mechanically actuated, positive displacement pump. Reciprocal movement of a string of sucker rods induces reciprocal movement of the pump for lifting production fluid to the surface. For example, a reciprocating rod lift system 20 of the prior art is shown in FIG. 1A to produce production fluid from a wellbore 10. As is typical, surface casing 12 hangs from the surface and has a liner casing 14 hung therefrom by a liner hanger 16. Production fluid F from the formation 19 outside the cement 18 can enter the liner 14 through perforations 15. To convey the fluid, production tubing 30 extends from a wellhead 32 downhole, and a packer 36 seals the annulus between the production tubing 30 and the liner 14. At the surface, the wellhead 32 receives production fluid and diverts it to a flow line 34.
The production fluid F may not produce naturally reach the surface so operators use the reciprocating rod lift system 20 to lift the fluid F. The system 20 has a surface pumping unit 22, a rod string 24, and a downhole rod pump 50. The surface pumping unit 22 reciprocates the rod string 24, and the reciprocating string 24 operates the downhole rod pump 50. The rod pump 50 has internal components attached to the rod string 24 and has external components positioned in a pump-seating nipple 31 near the producing zone and the perforations 15.
As best shown in the detail of FIG. 1B, the rod pump 50 has a barrel 60 with a plunger 80 movably disposed therein. The barrel 60 has a standing valve 70, and the plunger 80 is attached to the rod string 24 and has a traveling valve 90. For example, the traveling valve 90 is a check valve (i.e., one-way valve) having a ball 92 and seat 94. For its part, the standing 70 disposed in the barrel 60 is also a check valve having a ball 72 and seat 74.
As the surface pumping unit 22 in FIG. 1A reciprocates, the rod string 24 reciprocates in the production tubing 30 and moves the plunger 80. The plunger 80 moves the traveling valve 90 in reciprocating upstrokes and downstroke. During an upstroke, the traveling valve 90 as shown in FIG. 1B is closed (i.e., the upper ball 92 seats on upper seat 94). Movement of the closed traveling valve 90 upward reduces the static pressure within the pump chamber 62 (the volume between the standing valve 70 and the traveling valve 90 that serves as a path of fluid transfer during the pumping operation). This, in turn, causes the standing valve 70 to unseat so that the lower ball 72 lifts off the lower seat 74. Production fluid F is then drawn upward into the chamber 62.
On the following downstroke, the standing valve 70 closes as the standing ball 72 seats upon the lower seat 74. At the same time, the traveling valve 90 opens so fluids previously residing in the chamber 62 can pass through the valve 90 and into the plunger 80. Ultimately, the produced fluid F is delivered by positive displacement of the plunger 80, out passages 61 in the barrel 60. The moved fluid then moves up the wellbore 10 through the tubing 30 as shown in FIG. 1A. The upstroke and down stroke cycles are repeated, causing fluids to be lifted upward through the wellbore 10 and ultimately to the earth's surface.
The conventional rod pump 50 holds pressure during a pumping cycle by using sliding mechanical and/or hydrodynamic seals disposed between the plunger's outside diameter and the barrel's inside diameter. Sand in production fluids and during frac flowback can damage the seals. In particular, the differential pressure across the seals causes fluid to migrate past the seals. When this migrating fluid contains sand, the seals can become abraded by the sand so the seals eventually become less capable of holding pressure. Overtime, significant amounts of sand can collect between the plunger and the barrel, causing the plunger to become stuck within the barrel.
Production operations typically avoid using such a rod pump in wellbores having sandy fluids due to the damage that can result. However, rod pumping in sandy fluids has been a goal of producers and lift equipment suppliers for some time. To prevent sand damage, screens can be disposed downhole from the pump 50 to keep sand from entering the pump 50 altogether. Yet, in some applications, using a screen in such a location may not be feasible, and the screen and the rathole below can become fouled with sand. In other application, it may actually be desirable to produce the sand to the surface instead of keeping it out of the pump 50.
One solution to deal with sandy fluids uses extra tight seals in the pump 50 to exclude the sand. In pumping operations, however, there will always be some fluid leakage due to the pressure differential so eventually the sand will wear the seal. Extra loose hydrodynamic seals with long sealing surfaces are sometimes used to let sand pass. These long, loose hydrodynamic seals can extend the life of the pump because the longer seals can accommodate more damage than conventional rod pumps. However, damage still occurs; there is just more sacrificial surface to accept the damage. Thus, the life of the pump is extended even though damage continues.
Another solution to deal with sandy fluids shown in FIG. 2A uses a rod pump 50 as disclosed in U.S. Pat. No. 2,160,811. As before, the rod pump 50 has a plunger 80 disposed in a barrel 60 and has a standing valve 70 and a traveling valve 90. An upper sealing zone 84a between the plunger 80 and barrel 60 has hard metal rings 85 that engage inside the barrel 60. A lower sealing zone 84b uses the sliding cooperation between the barrel 60 and the plunger 80 to form a fluid seal. A chamber 86 is disposed between the two sealing zones 84a-b to deal with sand that may collect uphole of the plunger 80. This chamber 86 is maintained in communication with the interior 82 of the plunger 80 using circumferentially spaced ports 83.
During a downstroke of the plunger 80, the chamber 86 decreases in volume, and fluid displaces from the chamber 86 through the ports 83 and into the interior 82 of the plunger 80. Thus, any sand and silt that may have entered the chamber 86 through the upper sealing zone 84a is discharged into the plunger 80 to be removed with the main body of fluid. In this way, the sand or silt is prevented from reaching the lower sealing zone 84b and causing damage during a subsequent upstroke.
In a related solution to the rod pump 50 of FIG. 2A, a sand snare chamber can be used in the rod pump. For example, the Harbison-Fischer Sand-Pro® pump disclosed in U.S. Pat. Nos. 7,686,598 and 7,909,589 has a plunger with a sand snare chamber defined in its walls to catch the sand. (SAND-PRO is a registered trademark of Harbison-Fischer, Inc. of Crowley, Tex.) FIG. 2B shows an example of such a rod pump 50 having a sand snare chamber 100.
Again, the pump 50 has a barrel 60 with a plunger 80 located therein and has standing and traveling valves 70 and 90. The plunger 80 has a first portion 83 having a first seal 84a with the barrel 60, and the plunger 80 has a third portion 87 having a second seal 84b with the barrel 60. The first seal 84a has resilient members, while the second seal 84b is a fluid seal. An opening 81 at the top of the plunger 80 allows lifted fluid to pass up the barrel 60 and the production tubing (not shown) to be produced.
In between the first and second portions 83 and 87, the plunger 60 has a second portion 85 that forms a balancing chamber 86 between the barrel 60 and the plunger 80. The plunger's second portion 85 also has an opening 88 to allow communication between the plunger's interior 82 and the balancing chamber 86. A wall 89 is located relative to the opening 88 and forms a sand snare chamber 100 between the balancing chamber 86 and the plunger interior passage 82.
To pump fluid from a sandy well, the plunger 80 reciprocates with respect to the barrel 60. Pressure equalizes across the first seals 84a by venting pressure from inside of the plunger 82 to outside of the plunger 80 in the balancing chamber 86 between the two seals 84a-b. In the meantime, the pump 50 uses the wall 89 to capture sand from the fluid exiting the opening 88 in the sand snare chamber 100. This collection isolates the sand from the sets of seals 84a-b to reduce wear.
Unfortunately, the sand snare chamber 100 on the pump 50 has some drawbacks. For example, the volume available to collect sand can be limited. In addition, the chamber 100 can create turbulence during pumping which can tend to keep the sand flushed out of the sand snare chamber 100 and into the sealing areas 84a-b. 
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.